I'm not going to argue whether or not people are "born that way", because some probably are and some probably aren't. That's not the issue. Marriage is, and has been throughout history, an union of man and woman for the purpose of creating a family. They may or may not have children, nonetheless they have, through marriage, created that basic unit of society.
Now, if we want to give the same "rights" to other, non-marital couples, fine. Write up the laws and pass them. Don't try to change the definition of marriage, because that is more an effort to weaken that basic unit of society than it is to ensure "equal rights".